A loud bang heard in the early hours of Sunday in London and south east England was caused by two RAF jets scrambled after communications was lost with an aircraft in UK airspace.
A sound similar to an explosion at around 4.20am led to Hertfordshire Fire and Rescue receiving a "large number" of calls.
People living in Essex, west London and Stevenage were among those tweeting to say they had been woken up by the noise.
Scotland Yard said: "The loud bang heard throughout north London and surrounding areas was the result of a sonic boom from RAF planes. There is no cause for concern."
An RAF spokesperson said: "Two Typhoon fighter aircraft from RAF Coningsby were scrambled at 0409 this morning, as part of the UK's Quick Reaction Alert procedures, after an aircraft lost communications in UK airspace.
"The aircraft was intercepted and its communications were subsequently re-established.
"The Typhoons are returning to their base."
